1. Field of the Invention
This invention relates generally to sonar equipment and in particular to an acoustical energy absorbing baffle for minimizing sound reflection, and providing isolation from noise producing sources.
2. Description of the Prior Art
In the sonar art the desire for improved detection ranges and localization accuracy has brought about an increase in the size of sonar arrays. This, in turn, has raised the need for improved ways of mitigating the effect of background noise, such as the ambient noise of the sea, and the ship's own self noise. Ambient sea noise arises principally from the wave motion of the sea, from marine creatures, and from the radiated noise of ocean shipping. Ambient noise is usually considered to be isotropic, that is, non-directional. Self noise is caused by the flow of water over the hydrophones and by specific machinery noise generated by the ship on which the hydrophones are carried. Self noise is usually anisotropic or directional in nature. Both types can mask an incoming signal and thus present a problem to sonar system performance.
Acoustic baffles are often used with sonar arrays to provide discrimination against noise sources in certain directions as well as to alter the shape of the array's directivity pattern. The baffle can thus shield the sonar array from the ship's own self noise, as well as reduce the level of ambient noise by making the array more directive. Many baffle designs have been tried, although none have been wholly successful.
For instance, it is known that certain cellular, air trapping, sponge materials, if placed between the hydrophone and the ship, will produce an acoustical impedance mismatch which can reflect ship generated noise. But incident sonar signal energy is likewise reflected. Since air has a lower acoustical impedance than water, these reflections will be out of phase with the incident sonar waves and will add destructively to cancel the incident signal. One prior art solution to this cancellation problem is to place a steel signal conditioning plate between the sponge material and the hydrophone. Since steel has a higher impedance than water, and if sufficiently thick in respect to wavelength, signal energy is reflected in phase with incident energy adding constructively. At present day sonar wavelengths, the steel plate must be at least on the order of three-fourth inches thick in order to be acoustically visible. Thus weight is a significant problem. Another problem attendant to the reflective baffle is that, being designed to reflect acoustical energy, the baffle actually increases the vessel's visibility to other sonar detectors. This is particularly undesirable in submarine applications, where visibility should ideally be reduced, not increased.
Another prior art approach has been the attempt to absorb rather than reflect noise energy. With an absorptive baffle the object is to match the baffle's impedance to the medium's impedance, so noise energy will propagate freely into the baffle without reflection. Once inside the baffle ideally all noise energy is trapped and dissipated. However, as is known, perfect impedance matching is difficult to achieve over a broad band of frequencies and a wide range of hydrostatic pressures. For many baffle materials the response characteristics change dramatically with depth of submersion. For instance, neoprene rubber composites becomes less and less absorptive as it compresses under the increasing pressures of greater and greater oceanic depths. Furthermore, vibrational modes can exist within the baffle which introduce frequency dependent anomolies or non-linearities in the array response. These vibrational modes or resonances cause some energy to be re-radiated instead of dissipated.
Aside from their utility in connection with isolating sonar arrays from the effects of background noise, acoustical baffles are receiving increased attention for their utility as an anechoic hull coating for submarines and other vessels. Again, weight is a prime consideration, and the anechoic coating should provide relatively constant echo reduction over a wide range of frequencies and independent of the depth of submersion. Prior art techniques have far from met these requirements.
The present invention overcomes these difficulties and achieves a practical, light weight, pressure tolerant absorptive baffle that can be used with a sonar array. In addition, the invention also provides an anechoic baffle which may be placed on a vessel's hull surface to render the vessel less visible to sonar detection.